Field of the Invention
The present invention relates to an image estimating method, a non-transitory computer readable medium, and an image estimating apparatus, for estimating an image at an arbitrary focus position.
Description of the Related Art
In the virtual slide system, a digital image-pickup apparatus called a virtual slide is used to obtain a digital image of an object. In the medical field, an object is generally formed as a prepared specimen in which a sample is covered and fixed by an optical element (a cover glass). The virtual slide includes a microscope optical system, an image sensor, and an information processor, converts a prepared specimen into a digital image, and stores its data. Since this type of device stores only a digital image of the prepared specimen, a viewable image after the pickup is only an image at a focus position used for the pickup. Since a doctor frequently needs to determine a three-dimensional structure of the sample based on a series of images having different focus positions, it is necessary to capture a plurality of (stacked) images at different focus positions.
Acquiring many images requires a remarkably long image pickup time and a large data amount. Thus, minimizing the image pickup number is demanded. On the other hand, the excessively reduced image pickup number may result in a failure to provide an image at a focus position desired by a doctor in his diagnosis. In order to reconcile these two requirements, i.e., the reduced image pickup number and a provision of an image at an arbitrary focus position, there has been proposed a method for estimating an image at a necessary focus position through image processing.
Japanese Patent Laid-Open No. (“JP”) 2001-223874 proposes an image estimating method that applies a defocus filter in an optical system to images acquired at a plurality of focus positions. Kenji Yamazoe and Andrew R. Neureuther, “Modeling of through-focus aerial image with aberration and imaginary mask edge effects in optical lithography simulation,” Applied Optics, Vol. 50, No. 20, pp. 3570-3578, 10 Jul. 2011, U.S.A., discloses an approximately image estimating method that represents an image with a function of a focus position z and performs a polynomial expansion of z.
In order to minimize the image pickup number, it is necessary to widen an interval between two adjacent focus positions. However, the image estimating precision may lower when the image acquiring interval becomes excessively wide. In order to provide an image demanded by a doctor using a smaller number of images, the image acquiring interval may be maximized in a range in which the estimation precision is guaranteed.
Yamazoe et al. supra, and JP 2012-42525 propose setting an image acquisition interval to a value determined by the optical system. C. J. R. Sheppard, Min Gu, Y. Kawata and S. Kawata, “Three-dimensional transfer functions for high-aperture systems,” Journal of the Optical Society of America A, Vol. 11, No. 2, pp. 593-598, 1 Feb. 1994, U.S.A. describes a calculation method of a cutoff frequency.
The method disclosed in JP 2001-223874 needs to previously identify a defocus filter in the optical system, and requires a complicated preliminary measurement etc. In addition, this method is inapplicable to a partial coherent imaging system as in a microscope. The method disclosed in Yamazoe et al. supra, uses the polynomial of z as the function used for the expansion, and obtains an image as an approximated solution. The polynomial expansion up to a high order becomes necessary to improve the precision of the approximation, but then an extremely long calculation time is required. Moreover, the image acquiring interval determined only based on the information of the optical system depends upon the finest resolving power in the optical system. As a result, images are obtained at excessively narrow intervals, and a data capacity reducing effect becomes lower.